1. Field of the Invention
This invention relates generally to the field of well logging. More particularly, the invention concerns improved methods and systems in which an antenna system having transverse or tilted magnetic dipoles is used to estimate formation anisotropic resistivity, providing reduced borehole effects in vertical and small deviation wells. This invention is applicable to induction or propagation type measurements, i.e., at low and high frequencies.
2. Description of Related Art
Induction and propagation logging techniques have been employed in exploration operations for many years to measure the electrical conductivity (or its inverse, resistivity) of subsurface formations. These techniques entail deployment of antenna systems into a borehole to emit electromagnetic (EM) energy through the borehole fluid (also referred to herein as mud) and into the subsurface formations. Conventional antennas are formed from coils of the cylindrical solenoid type comprised of one or more turns of insulated conductor wire wound around a support. These antennas are typically operable as sources and/or sensors.
In operation, a transmitter antenna is energized by an alternating current to emit EM energy. The emitted energy interacts with the mud and the formation, producing signals that are detected and measured by one or more of the antennas. The detected signals are usually expressed as a complex number (phasor voltage) and reflect the interaction with the mud and the formation. By processing the detected signal data, a profile of the formation and/or borehole properties is determined.
A coil carrying a current can be represented as a magnetic dipole having a magnetic moment proportional to the current and the area encompassed by the coil. The direction and strength of the magnetic dipole moment can be represented by a vector perpendicular to the area encompassed by the coil. In conventional induction and propagation logging systems, the antennas are typically mounted on a support with their axes along the longitudinal axis of the support. Thus, these instruments are implemented with antennas having longitudinal magnetic dipoles (LMD). When such an antenna is placed in a borehole and energized to transmit EM energy, currents flow around the antenna in the borehole and in the surrounding formation. There is no net current flow up or down the borehole.
An emerging technique in the field of well logging is the use of instruments incorporating antennas having tilted or transverse coils, i.e., where the coil""s axis is not parallel to the support axis. These instruments are thus implemented with antennas having a transverse or tilted magnetic dipole (TMD). One particular implementation uses a set of three coils having non-parallel axes (referred to herein as tri-axial). The aim of these TMD configurations is to provide EM measurements with directional sensitivity to the formation properties. Logging instruments equipped with TMDs are described in U.S. Pat. Nos. 6,163,155, 6,147,496, 5,757,191, 5,115,198, 4,319,191, 5,508,616, 5,757,191, 5,781,436, 6,044,325, 4,264,862 and 6,147,496.
If a transmitter is placed in a homogeneous medium, currents will flow in paths surrounding the transmitter. When a borehole is added, these current paths are distorted. These currents induce a voltage in a receiver displaced from the transmitter. This voltage is an indication of the resistivity of the formation. If instead of a homogeneous medium, we include a borehole, then the current paths are altered and hence the received voltage is different from what would be measured in the absence of a borehole. This difference is called the xe2x80x9cborehole effect.xe2x80x9d The difference in borehole effect between a LMD-based tool and a TMD-based tool is due to the difference between the distortion of the currents in the presence of a borehole.
A particularly troublesome property of the TMD is the extremely large borehole effect that occurs in high contrast situations, i.e., when the mud in the borehole is much more conductive than the formation. When a TMD is placed in the center of a borehole, there is no net current along the borehole axis. A TMD can be eccentered in a borehole in two possible orientations, which we will call parallel and perpendicular eccentering as shown in FIG. 1. In parallel eccentering, the TMD is in a direction parallel to the direction of the magnetic moment. The symmetry of this situation insures that there is still no net current along the borehole axis. However, when a TMD is eccentered in a direction perpendicular to the direction of the magnetic moment, axial currents are induced in the borehole. In high contrast situations these currents can flow for a very long distance along the borehole. When these currents pass by TMD receivers, they can cause undesired signals that are many times larger than would appear in a homogeneous formation without a borehole, resulting in erroneous measurements.
The techniques for processing measured signal data to determine formation parameters involve a number of mathematical calculations. U.S. Pat. No. 4,302,722 (assigned to the present assignee) describes techniques for determining formation conductivity and anisotropy parameters from the measurements. U.S. Pat. Nos. 5,781,436, 5,999,883 and 6,044,325 describe methods for producing estimates of various formation parameters from tri-axial measurements. U.S. Pat. No. 5,041,975 (assigned to the present assignee) describes a technique for processing signal data from downhole measurements in an effort to correct for borehole effects. U.S. Pat. No. 5,058,077 describes a technique for processing downhole sensor data in an effort to compensate for the effect of eccentric rotation on the sensor while drilling.
It is desirable to have a simplified technique for measuring anisotropic resistivity using well tools implemented with TMDs. Thus there remains a need for improved measurement techniques that reduce or eliminate borehole effects associated with the flow of undesired axial currents along the borehole.
Systems and methods are provided for determining subsurface formation properties from EM measurements. The measurements are acquired within the formation with a well tool adapted with a TMD antenna system.
One aspect of the invention provides a method for determining a property of a subsurface formation using an antenna system disposed within a borehole traversing the formation. The method comprises determining when a magnetic dipole moment of a transmitter antenna of the system is oriented substantially perpendicular with respect to the wall of the borehole; determining the coupling between a receiver antenna of the system and the transmitter antenna when the dipole moment is in the substantially perpendicular orientation; and using the coupling between the antennas to determine the formation property.
Another aspect of the invention provides a method for determining a property of a subsurface formation from signal data obtained with a transmitter antenna and a receiver antenna disposed within a borehole traversing the formation. The method comprises determining the orientation of a magnetic dipole moment of the transmitter antenna using the signal data; determining the coupling between the transmitter antenna and the receiver antenna with the dipole moment oriented substantially perpendicular with respect to the wall of the borehole; and using the coupling between the antennas to determine the formation property.
Another aspect of the invention provides a well logging system including a well tool adapted for deployment through a borehole traversing an earth formation and an apparatus adapted for coupling to the well tool, the well tool having a longitudinal axis and including a transmitter antenna and a receiver antenna disposed thereon. The apparatus comprises means for determining when a magnetic dipole moment of the transmitter antenna is oriented substantially perpendicular with respect to the wall of the borehole; means for determining the coupling between the transmitter antenna and the receiver antenna with the dipole moment in the substantially perpendicular orientation; and means for determining a property of the formation using the coupling between the antennas.